Proxima Centauri b , also known as "Proxima b" or "Alpha Centauri Cb", is a terrestrial extrasolar planet orbiting within the habitable zone of the red dwarf star Proxima Centauri, the closest star to the Sun and part of the Alpha Centauri star system. It is located around 4.2 light years (40 trillion kilometers/25 trillion miles) away from Earth in the constellation Centaurus. It is the closest known extrasolar planet to the Solar System, along the disputed planets Proxima Centauri c and d.
Proxima Centauri b orbits its host star at a distance of around 0.05 AU (7,500,000 km; 4,600,000 mi), and has an orbital period of around 11.2 Earth days. The other properties of the planet are not fully known, but it is estimated to have a mass of at least 1.07 Earth masses, and a radius only slightly larger than that of Earth. The planet is also known to orbit within the habitable zone of its star, but it is not known whether or not it has an atmosphere. As Proxima Centauri is a flare star, the planet is subject to extreme stellar wind pressures, more than 2,000 times those experienced by Earth from the solar wind, which could potentially strip the atmosphere off the planet. The planet's proximity to Earth brings a unique opportunity for robotic space exploration; Breakthrough Starshot is an example of such a project.
The discovery of the planet was announced on August 24, 2016 by the European Southern Observatory. The planet was found using the radial velocity method, where periodic Doppler shifts of spectral lines of the host star suggest an orbiting object. From these readings, the radial velocity of the parent star relative to the Earth is varying with an amplitude of about 2 meters (7 feet) per second.
Discovery[]
Proxima Centauri had been the target for extrasolar planet searches for many years before the discovery of Proxima Centauri b. Studies in 2008 and 2009 ruled out the possibility of larger-than-Earth planets in the habitable zone.
The first indications of an extrasolar planet were found in 2013 by Mikko Tuomi of the University of Hertfordshire from archival observation data. To confirm the possible discovery, the European Southern Observatory launched the "Pale Red Dot" project in January 2016.
On August 24, 2016, the team of 31 scientists from all around the world, led by Guillem Anglada-Escudé of Queen Mary University of London, confirmed the existence of Proxima Centauri b through a peer-reviewed article published by Nature. The measurements were done using two spectrographs, HARPS on the ESO 3.6 m Telescope at La Silla Observatory and UVES on the 8-metre Very Large Telescope. The peak radial velocity of the host star combined with the orbital period allowed for the minimum mass of the planet to be calculated. The chance of a false positive detection is less than one in ten million.
In 2022, another extrasolar planet, Proxima Centauri d, was confirmed. Another candidate, Proxima Centauri c, was reported in 2020, but its existence has been disputed.
Details[]
Distance, orbital parameters, and age[]
Proxima Centauri b is the closest known extrasolar planet to Earth, being about 4.2 light years away. It completes an orbit around Proxima Centauri every 11.2 Earth days, at a distance of about 0.049 AU, which is over 20 times closer to Proxima Centauri than Earth is to the Sun. It is unclear whether the planet has an eccentricity, but it is unlikely to have any obliquity. The age of the planet is not known; the Proxima Centauri star itself may have been captured by Alpha Centauri and is thus not necessarily the same age as it. Proxima Centauri b is unlikely to have any stable orbits for extrasolar moons.
Mass, radius, and temperature[]
The minimum mass of Proxima Centauri b was calculated to be 1.17 Earth masses in 2020, which would be the actual mass if its orbit was seen edge-on from the Earth; this figure has recently been updated to being a minimum of at least 1.07 Earth masses. All estimates, however, are dependent on the inclination of the planet's orbit, and thus they may be either underestimates or overestimates. This makes it similar to Earth in terms of mass, but the radius of the planet is poorly understood and hard to estimate. The planet could be a Mercury-like planet with a large core (which would require certain conditions during the planet's history), to an extremely water-rich planet.
It is likely that Proxima Centauri b developed differently from the Earth, and as such has less water, stronger impacts, and an overall faster development, assuming it formed at its current distance from the star. However, it likely did not form at its current distance from Proxima Centauri as the amount of material in the protoplanetary disk would be insufficient. Instead, it or fragments formed at larger distances and then migrated to the current orbit of Proxima Centauri b. Depending on the nature of the precursor material, it may be rich in volatiles . A number of different formation scenarios are possible, many of which depend on the existence of other planets around Proxima Centauri and which would result in different compositions.
Tidal locking[]
Proxima Centauri b is likely tidally locked to its star, meaning that the same side of the planet would always face Proxima Centauri. It is unclear and disputed whether or not life can form in this state, as a 1:1 tidal lock would lead to an extreme climate with only part of the planet habitable (the "terminator line").
However, it's possible the planet may not be tidally locked. If the eccentricity of the planet is higher than 0.1-0.06, it would likely enter a Mercury-like 3:2 resonance or higher-order resonances such as 2:1. Additional planets around Proxima Centauri and interactions with Alpha Centauri could excite higher eccentricies.
Host star[]
The planet orbits a star named Proxima Centauri, an M-type red dwarf star located 4.2 light years away from the Sun. The star has a mass of 0.12 Solar masses and a radius of 0.14 Solar radiuses. It has a surface temperature of 3042 K, and is about 4.85 billion years old. For comparison, the Sun is about 4.6 billion years old, and has a surface temperature of 5778 K. Proxima Centauri rotates once roughly every 83 days, and has a luminosity of about 0.0015 Solar luminosity. Like the two larger stars in the triple star system, Proxima Centauri is rich in metals compared to the Sun, something which is not normally found in low-mass stars like Proxima. Its metallicty ([Fe/H]) is 0.21, or 1.62 times the amont found in the Sun's atmosphere.
Even though Proxima Centauri is the closest star to the Sun, it is not visible to the unaided eye from Earth due to its low luminosity (apparent magnitude of 11.13).
Proxima Centauri is a flare star, which means it undergoes occasional dramatic increases in brightness and high-energy emissions because of magnetic activity that would create large solar storms. The surface of Proxima Centauri b could possibly be irradiated if it does not possess a strong magnetic field or a protective atmosphere.
Orbit[]
Proxima Centauri b orbits its host star about every 11.186 days, at a semi-major axis distance of approximately 0.05 astronomical units (7,000,000 km; 5,000,000 mi), which means the distance from the exoplanet to its host star is one-twentieth of the distance from the Earth to the Sun. Comparatively, Mercury, the closest planet to the Sun, has a semi-major axis distance of 0.39 AU. Proxima Centauri b receives about 65% of the amount of radiative flux from its host star that the Earth receives from the Sun - for comparison, Mars receives about 43%. Most of the radiative flux from Proxima Centauri is in the infrared spectrum. In the visible spectrum, the exoplanet only receives 2.1% of the light Earth does - for comparison Jupiter receives 3.7% and Saturn receives 1.1%. So it would not typically get much brighter than twilight anywhere on Proxima Centauri b's surface. The maximum illumination of horizontal ground by twilight at sunrise is about 400 lux, while the illumination of Proxima Centauri b is about 2700 lux with quiet Proxima. Also, Proxima has flares. The brightest flare so far observed increased the visual brightness of Proxima about 8 times, which would be a large change from previous level but, at about 17% the illumination of Earth, not very strong sunlight. However, because of its tight orbit, Proxima Centauri b receives about 400 times more X-ray radiation than the Earth does.
Habitability[]
Artist conception of Proxima Centauri B's surface.
The habitability of Proxima Centauri b is not established and highly debated. The planet is subject to more than 2,000 times the stellar wind pressures than those experienced by Earth from the solar wind. This radiation and stellar winds would likely blow any atmosphere away, which would leave the undersurface as the only potentially habitable location on the planet.
However, the planet orbits within the habitable zone of Proxima Centauri, the region in which, with the correct planetary conditions and atmospheric properties, liquid water might exist on the surface of the planet. The host star, with about an eighth of the mass of the Sun, has a habitable zone between ∼0.038-0.053 AU. In October 2016, researchers at France's CNRS research institute stated that there is a considerable chance of the planet harboring surface oceans and having a thin atmosphere. However, unless the planet transits in front of its star from the perspective of Earth, it is difficult to test these hypotheses.
Even though Proxima Centauri b is in the "habitable zone", the planet's habitability has been questioned because of several potentially hazardous physical conditions. For instance, the planet is close enough to its host star that it may be tidally locked. If this is the case, then it is expected that any habitable areas would be confined to the border region between the two extreme sides, referred to as the "terminator line", since it is only here that temperatures might be suitable enough for liquid water to exist. If the planet's orbital eccentricity is 0, this could result in synchronous rotation, with one hot side permanently facing towards the star, while the opposite side is in permanent darkness and freezing cold. However, Proxima Centauri b's orbital eccentricity is not known with certainty, only that it is below 0.35 - potentially high enough for it to have a significant chance of being captured into a 3:2 spin-orbit resonance similar to that of Mercury, where Proxima Centauri b would rotate around its axis approximately every 7.5 Earth days, with about 22.4 Earth days elapsing between one sunrise and the next. Resonances as high as 2:1 are also possible.
The European Southern Observatory estimates that if water and an atmosphere are present, a far more hospitable environment would result. Assuming an atmospheric N2 pressure of 1 bar and ∼0.01 bar of CO2, in a world including oceans with average temperatures similar to those on Earth, a wide equatorial belt (non-synchronous rotation), or the majority of the sunlit side (synchronous rotation), would be permanently ice-free. A large portion of the planet may be habitable if it has an atmosphere thick enough to transfer heat to the side facing away from the star. If it has an atmosphere, simulations suggest that the planet could have lost about as much as the amount of water that Earth has due to the early irradiation in the first 100–200 million years after the planet's formation. Liquid water may be present only in the sunniest regions of the planet's surface in pools either in an area in the hemisphere of the planet facing the star or - if the planet is in a 3:2 resonance rotation - diurnally in the equatorial belt. All in all, astrophysicists consider the ability of Proxima Centauri b to retain water from its formation as the most crucial point in evaluating the planet's present habitability. The planet may be within reach of telescopes and techniques that could reveal more about its composition and atmosphere, if it has any.
On March 18, 2016, a superflare on Proxima Centauri was observed, unleashing 316,227,766,000 petajoules (316,227 petawatts) of energy, one of the most powerful flares ever observed for a star its size. The surface irradiation was estimated to be around 100 times what is required to kill even UV-hardy microorganisms, and would be lethal to all currently known life. Based on the rate of observed flares, total ozone depletion of an Earth-like atmosphere would occur within several hundred thousand years.
Sky view[]
An observer on the surface of Proxima Centauri b would see a very similar night sky to Earth's. However, a few key differences would be noticeable. Centaurus would no longer be the brightest star. The Sun would also be visible, and it would appear as a yellow star with an apparent magnitude of +0.5 in eastern Cassiopeia. The \/\/ shape of Cassiopeia would now become a /\/\/ shape.
Orion would generally become unchanged, except that Sirius would lie less than a degree from Betelgeuese, and it would be slightly fainter, with a magnitude of -1.2.
Procyon would be in the middle of Gemini, outshining Pollux, whereas both Vega and Altair would be shifted northwestward relative to Deneb, which has barely moved.
From Proxima Centauri b, Alpha Centauri AB would appear like two close bright stars with the combined apparent magnitude of -6.8
Formation[]
It is unlikely Proxima Centauri b originally formed in its current orbit since disk models for small stars like Proxima Centauri would contain less than one Earth mass of matter within the cental one AU at the time of their formation. This implies that either Proxima Centauri b was formed elsewhere in a manner still to be determined, or the current disc models for stellar formation are in need of revision.
Future observations[]
A team of scientists believe they may be able to image Proxima Centauri b, and probe the planet's atmosphere for signs of oxygen, water vapor, and methane, combing ESPRESSO and SPHERE on the VLT. The James Webb Space Telescope may be able to characterize the atmosphere of Proxima Centauri b, but there is no conclusive evidence for transits combing MOST and HATSouth photometry, giving it less than a 1 percent chance of being a transiting planet. Future telescopes (the Extremely Large Telescope, the Giant Magellan Telescope, and the Thirty Meter Telescope) could have the capability to characterize Proxima Centauri b.
The discovery of Proxima b was significant to Breakthrough Starshot, a proof of concept project aiming to send a fleet of miniature probes to the Alpha Centauri system. The project is led by research company Breakthrough Initiatives, and plans to develop and launch a fleet of miniature unmanned spacecraft called StarChips, which could travel at up to 20% of the speed of light. This means that it could arrive in the Proxima Centauri system in 20 years, and notify Earth about 4 years later.
2069 Alpha Centauri mission[]
In 2017, Breakthrough Initiatives and the European Southern Observatory (ESO) entered a collaboration to enable and implement a search for habitable planets in the nearby star system, Alpha Centauri. The agreement involves Breakthrough Initiatives providing funding for an upgrade to the VISIR (VLT Imager and Spectrometer for mid-Infrared) instrument on ESO’s Very Large Telescope (VLT) in Chile.